Method for locally modifying electronic and optoelectronic properties of crystalline materials an devices made from such materials

ABSTRACT

An electronic or optoelectronic device fabricated from a crystalline material in which a parameter of a bandgap characteristic of said crystalline material has been modified locally by introducing distortions on an atomic scale in the lattice structure of said crystalline material and the electronic and/or optoelectronic parameters of said device are dependent on the modification of said bandgap is exemplified by a radiation emissive optoelectronic semiconductor device which comprises a junction ( 10 ) formed from a p-type layer ( 11 ) and an n-type layer ( 12 ), both formed from indirect bandgap semiconductor material. The p-type layer ( 11 ) contains an array of dislocation loops which create a strain field to confine spatially and promote radiative recombination of the charge carriers.

This invention relates to electronic and optoelectronic devicesfabricated from crystalline materials and a method of making same. Itfinds particular application in radiation emissive optoelectronicdevices made from semiconductor materials initially having an indirectbandgap.

Crystalline materials consist of arrays of atoms arranged in regularpatterns. The electrical charge associated with the component electronsand nuclear particles create within the crystal an electric field whichvaries locally with a periodicity determined by the spacing of the atomswithin the crystal.

Semiconductors and insulators formed of crystalline materials arecharacterised by so-called conduction and valence bands by way of whichcharge carriers propagate under the influence of an applied field. Theenergy separation of the conduction and valence bands is determined bythe nature and size of the constituent atoms of the crystal. In somematerials a minimum in the valence band may be opposite a maximum in theconduction band, whilst in others is displaced therefrom. These areknown respectively as direct and indirect bandgap materials.

The nature and magnitude of a material's band structure are parameterswhich influence the electronic and optoelectronic devices fabricatedtherefrom. For example, diodes made from semiconductors with widebandgaps will tend to have higher breakdown voltages because thesematerials will have fewer thermally-generated charge carriers at anygiven temperature and therefore will be less susceptible to avalancheeffects. Gallium arsenide will be a material of choice forradiation-generating devices because it has a direct bandgap. Silicon,on the other hand, has been considered fundamentally unsuitable for useas an emitter of radiation. This is because silicon is an indirectbandgap material in which fast, non-radiative recombination processescompletely dominate the much slower radiative recombination processes.Indeed in bulk silicon, at room temperature, radiation emission isalmost entirely absent.

With the continuing and rapid development of computer processors, theconstant demand for increased processing power and speed and reducedsize necessitates an ever increasing complexity of the interconnectingmetallisations. It is anticipated that this complexity will eventuallypresent an insurmountable obstacle to further development (the breakdownof Moore's Law) because electrons will spend a disproportionate amountof time in the metallisations instead of in the components theyinterconnect, thereby curtailing processing power and speed.

Optoelectronic circuits based on silicon technology offer a way forwardbecause optical coupling is many orders of magnitude faster thanconnections based on the diffusion of charge carriers. However, thisapproach requires development of an efficient room temperatureradiation-emissive device based on silicon. Clearly, such a device couldbe used to enhance the functionality of other silicon devices and couldlead to implementation of all-silicon integrated optoelectronic systems.

By introducing a strain field created by local distortions on an atomicscale in the structure of a semiconducting or insulating crystal, wehave found that it is possible locally to modify the properties of theassociated bandgap. One consequence of this is that it has provedfeasible to fabricate radiation-emitting devices from materials such assilicon.

A preferred mechanism for the creation of a strain field is theformation of an array of dislocation loops.

According to the present invention there is provided an electronic oroptoelectronic device fabricated from a crystalline material in which aparameter of a band structure characteristic of said crystallinematerial has been modified locally by introducing distortions on anatomic scale in the lattice structure of said crystalline material andwherein a desired electronic or optoelectronic parameter of said deviceis obtained as a result of said the modification of the band structure.

According to a particular aspect of the invention there is provided aradiation-emissive optoelectronic device comprising a junction formed,at least in part, from a region of p-type indirect bandgap semiconductormaterial and/or a region of n-type indirect bandgap semiconductormaterial, wherein said junction incorporates means effective, inoperation of the device, to confine spatially, and thereby promote,radiative recombination of charge carriers.

Said charge carrier confinement means is preferably a strain field.

In a preferred implementation of the invention, said strain field ispreferably created by intrinsic extended lattice defects such as anarray of dislocation loops.

Depending on the materials used, devices according to the invention mayemit radiation at different wavelengths in a range extending from theinfra-red to the ultra violet. In a preferred embodiment, the devicecomprises a p-n junction formed from a region of n-type silicon and aregion of p-type silicon. In operation, this device emits radiation inthe infra red, at a wavelength of about 11505 nm. As will bedemonstrated, a silicon-based device of this kind may be readilymanufactured on a commercial scale using processing techniques common toULSI technology.

Devices according to the invention have the additional advantage thatoptimum operating efficiency i.e. maximum total electroluminescenceintensity is achieved at or around room temperature. Thus, the devicesare particularly well suited for use in applications such as theaforementioned.

According to another aspect of the invention there is provided a methodof making a radiation-emissive optoelectronic semiconductor deviceincluding the steps of forming a junction having at least a region ofp-type indirect bandgap semiconductor material and/or a region of n-typeindirect bandgap semiconductor material, and incorporating in thejunction means effective, in operation of the device, to spatiallyconfine, and thereby promote radiative recombination of charge carriers.

In a preferred embodiment the junction is formed, at least in part, froma region of p-type indirect bandgap semiconductor material and a regionof n-type indirect bandgap material.

Embodiments of the invention are now described, by way of example only,with reference to the accompanying drawings of which:

FIG. 1 shows a schematic transverse sectional view through aradiation-emissive optoelectronic device according to the invention,

FIG. 2 is a plot of current I as a function of bias voltage V obtainedat room temperature from the device of FIG. 1,

FIG. 3 shows plots of integrated electroluminescence intensity as afunction of forward bias voltage V obtained from the device of FIG. 1operating at 80K, 180K and 300K,

FIG. 4 shows plots of electroluminescence intensity as a function ofwavelength obtained from the device of FIG. 1 operating at 80K, 140K,200K, 260K and 320K and

FIG. 5 shows a plot of integrated electroluminescence intensity as afunction of temperature derived from the plots of FIG. 4, and

FIGS. 6 a and 6 b are respectively a diagrammatic cross-section andblock diagram of an optoelectronic integrated circuit in accordance witha specific embodiment of the invention.

Referring now to FIG. 1, the radiation-emissive optoelectronic devicehas the form of a diode comprising a p-n junction 10 defined by a region11 of p-type silicon and a region 12 of n-type silicon. In thisimplementation of the invention, the p-type region 11 is doped withboron (B) and the n-type region 12 is doped with arsenic (As). However,it will be appreciated that other suitable dopants known to thoseskilled in the art could alternatively be used.

Ohmic contacts 13,14 are provided on the front and back surfaces 15,16of the device enabling a bias voltage to the applied across the junction10. In this embodiment, the ohmic contact 13 provided on the frontsurface 15 of the device is made from aluminium (Al) and the ohmiccontact 14 provided on the back surface 16 of the device is made fromeutectic gold/antimony alloy (AuSb). Contact 14 has a central window 17through which electroluminescence produced by the device can pass.

The junction region 10 incorporates a strain field. In this embodiment,the strain field is created by extended intrinsic lattice defects suchas an array of dislocation loops situated in the region 11 of p-typesilicon, as shown schematically in FIG. 1.

The effect of the strain field is locally to modify the structure of thesilicon bandgap. More specifically, the strain field around eachextended lattice defect gives rise to a three-dimensional potential wellwhich varies inversely as a function of distance from the core of thedislocation loop. It is believed that the combined effect of thepotential wells is to cause spatial confinement of mobile chargecarriers thereby significantly reducing their diffusion to point defectsin the silicon where fast, non-radiative recombination processes wouldotherwise take place. It has been found that the effect of a strainfield of the kind described is to suppress non-radiative recombinationof charge carriers, which is usually the dominant process, and topromote radiative recombination of charge carriers which, hitherto, hasbeen almost entirely absent in devices made from indirect bandgapmaterials such as silicon. As will be described in greater detailhereinafter, when a forward bias voltage is applied across junction 10significant amounts of electroluminescence are generated by the device.

Promotion of the radiative recombination process is enhanced if thearray A of dislocation loops is periodic (or nearly so) in the lateraldirections of junction; that is, in directions parallel to the interfaceof the p-type and n-type regions 11,12 of the junction. In thisembodiment, the array of dislocation loops has a periodicity of around100 nm and is located in region 11 at a depth of about 100 nm.

The device described with reference to FIG. 1 was fabricated byimplanting boron atoms into a device grade substrate of n-type siliconhaving a resistivity of 2-4 ohm-cm using a conventional ion implantationprocess. In this embodiment, the implantation dose was 1×10¹⁵ cm⁻² andthe implantation energy was 30 keV.

The implanted substrate was then annealed in a nitrogen atmosphere forabout 20 minutes at 1000° C. The ohmic contacts 13,14 were then appliedto the substrate by evaporation or by another suitable depositionprocess and sintered at 360° C. for about two minutes.

In this implementation, the implanted boron atoms serve dual functionsso as to reduce the number of processing steps; that is, the implantedboron atoms are used as dopant atoms defining the p-type region 11 ofthe junction and they are also used to create dislocations in thatregion. The subsequent annealing step activates the implanted dopantatoms and also leads to aggregation of the dislocations which causes therequired dislocation loop array to form.

In another implantation, the array of dislocation loops is formedindependently of the doping process by separately implanting a differentspecies of atom e.g. silicon atoms. Again, an implantation energy ofabout 30 keV is used.

In both the foregoing implementations, the fabrication process istailored deliberately to introduce dislocations into the substrate (astep usually considered undesirable) to enable the required array ofdislocation loops to form during the annealing step.

It will be appreciated that the techniques employed in the describedfabrication processes (i.e. ion implantation, evaporation, annealing)are entirely compatible with existing USLI technology. Accordingly, thedevice described could be readily fabricated on standard fabricationlines.

In order to investigate the operating characteristics of the describeddevice a bias voltage V was applied across the ohmic contacts 13,14 andthe electrical current I between the contacts was measured. FIG. 2 is aplot showing the variation of current I as a function of voltage V anddemonstrates that the device exhibits the characteristic behaviour of adiode.

To investigate radiation emissivity, the device was mounted in a holderinside a continuous flow, liquid nitrogen cryostat. Electroluminescenceproduced by the device was focused into a conventional half metrespectrometer and detected using a liquid nitrogen cooled germanium p-i-ndetector.

FIG. 3 shows plots of the integrated or total electroluminescenceintensity as a function of forward bias voltage detected at 80K, 180Kand 300K and demonstrates how the onset of electroluminescence isobserved as the diode turns on. FIG. 4 shows the fullelectroluminescence spectra obtained at 80K, 140K, 200K, 260K and 320Kand FIG. 5 shows a plot of the integrated or total electroluminescenceintensity as a function of temperature derived from theelectroluminescence spectra of FIG. 4. Referring to FIG. 4, the lowtemperature spectrum, obtained at 80K, exhibits the structural featuresexpected for radiative emission at the silicon band edge. The roomtemperature spectrum peaks at the wavelength 1150 nm and has a fullwidth at half maximum (FWHM) of 80 nm. Referring to FIG. 5, it can beseen that the integrated or total electroluminescence steadily increasesas a function of temperature, an effect thought to be attributable tothe increasing role of phonon coupling in the radiative recombinationprocess. Accordingly, optimum emissivity is achieved at room temperatureand above. This is in sharp contrast to known systems proposed forradiation emission in silicon, for which electroluminescence quenchesstrongly with increasing temperature making a practical room temperaturedevice problematical.

It will be understood that although the device described with referenceto FIGS. 1 to 5 comprises a silicon homojunction, the present inventionembraces devices comprising homojunctions made from other indirectbandgap materials including silicon alloys. For example, devicescomprising homojunctions made from materials ranging from 100% Ge,through germanium/silicon alloys (Ge/Si), through 100% Si throughsilicon carbide (SiC) alloys, will be emissive of radiation at differentrespective wavelengths in a range extending from the near infra-red(including the regions at and around 1.3 μm and 1.5 μm) up to theultraviolet.

It will be understood that the present invention also embraces devicescomprising heterojunctions e.g. silicon and germanium.

It will be understood that throughout this specification the expressionp-n junction is intended to embrace a p-i-n junction in which a regionof intrinsic semiconductor material (e.g. intrinsic silicon) issandwiched between the p-type and n-type regions of the junction.

It is envisaged that radiation-emissive optoelectronic devices accordingto the invention will have wide applicability; particularly, though notexclusively, in applications requiring efficient room temperatureelectroluminescence. All-silicon devices according to the invention mayfind application as radiation sources in all-silicon integrated opticalsystems.

Typically, devices fabricated in accordance with this invention may beincorporated in optoelectronic integrated circuits. Such circuits mayincorporate regions exhibiting a photonic bandgap.

Referring now to FIGS. 6 a and 6 b, a planar optoelectronic integratedcircuit 61 comprises a silicon-based optical emitter 63, which may beeither a non-coherent light-emitting diode (LED) or a laser, coupled byway of a planar waveguide 64 to a silicon-germanium (SiGe) opticaldetector 65. The circuit is formed on a silicon substrate 67incorporating a buried silicon dioxide layer 69. The optical emitter isformed in an n-type region 71 having a p-type region 73 formed by ionimplantation with an array of dislocation loops 75 adjacent a p-njunction 77. Metallic contacts 79,81 are respectively attached to p+ andn+ surface areas.

The waveguide region 83 is bounded by the buried oxide layer 69 and aregion 85 containing an array of regions of lower refractive index tocreate a photonic band gap, thereby modifying the wavelengthtransmission characteristics of the waveguide. Under some circumstances,these transmission characteristics may be further modified by theincorporation of an array of dislocation loops within the photonic bandgap region.

The optical detector 65 includes a SiGe p-n junction region 87 formedeither by ion beam synthesis involving implanation of germanium into thesilicon substrate or selective epitaxy. Contacts 89,91 are made to theactive region. A local p+ implantation 93 facilitiates this.

The individual components of the integrated circuit are separated byoxide-filled isolating trenches 95,97,99,101.

Vertical integration could also be implemented using an adapation ofthese techniques, leading to three-dimensional integration capability.Optically active regions incorporating impurities such as erbium orother rare earths or, for example, carbon with quasi-stable transitionsmay also be used as a building-block in the fabrication of theintegrated circuits or even discrete components.

In yet another application, a radiation-emissive optoelectronic deviceaccording to the invention may be used as the radiation source of aninjection laser. In this application, the device may include one or moreadditional region of p-type and/or n-type and/or undoped semiconductormaterial arranged to provide carrier population inversion and/or todefine an optical cavity for the emitted radiation. Examples of suchlasers include the separate confinement heterostructure (SCH) laser andthe large optical cavity (LOC) laser. In another example, the devicecomprises a junction formed, at least in part from a region of p-typeindirect bandgap semiconductor material and/or a region of n-typeindirect bandgap semiconductor material, wherein said junctionincorporates means effective, in operation of the device, to confinespatially, and thereby promote radiative recombination of chargecarriers. An injection laser may incorporate such a radiation-emissiveoptoelectronic device. Moreover, such an injection laser may incorporatesuch a radiation-emissive optoelectronic device that includes one ormore region of n-type and/or p-type and/or undoped semiconductormaterial arranged to provide carrier population inversion and/or todefine an optical cavity for radiation emitted by the device.

Although annealing has been described as having been carried out at atemperature of 1000° C. for twenty minutes, alternative combinations oftime and temperature may be employed. The combination will be such as toform and stabilise the strain-inducing local modifications to thecrystal structure and may even embrace flash annealing at temperaturesclose to the melting point of the material.

Under some circumstances it may be desirable to perform multiple ionimplantations to form arrays of dislocation loops or otherstrain-inducing microstructures at different distances from the surfaceof the crystal. Such a technique imparts a further degree of freedom inthe topography of the resulting devices.

The technique may be utilised to modify or fine-tune the characteristicsof devices produced by other methods, for instance, photonic bandgapregions produced by arrays of material of different refractive index.

The technique may also be confined locally in a substrate by usingphotolithographic masking processes.

1. A radiation-emissive optoelectronic device fabricated from a crystalline material in which a parameter of a band structure characteristic of said crystalline material has been modified locally by introducing distortions on an atomic scale in the lattice structure of said crystalline material forming dislocation loops, and wherein a desired electronic or optoelectronic parameter of said device is obtained as a result of the modification of the band structure, said device comprising a junction formed, at least in part from a region of p-type indirect bandgap semiconductor material and/or a region of n-type indirect bandgap semiconductor material, wherein said junction incorporates a strain field, created by an array of dislocation loops effective, in operation of the device, to confine spatially, and thereby promote radiative recombination of charge carriers, wherein said array of dislocation loops has substantial spatial periodicity in at least one lateral direction of the junction.
 2. A device as claimed in claim 1 wherein said region of p-type indirect bandgap semiconductor material is p-type silicon and said region of n-type indirect bandgap semiconductor material is n-type silicon, and said array of dislocation loops is formed in said region of p-type silicon and has a spatial periodicity of about 100 nm in the lateral directions of the junction. 